1. Field of the Invention
The present invention relates to a noise reduction technique in an image capture apparatus that captures an object image using a solid-state image sensor.
2. Description of the Related Art
In recent years, image capture apparatus which use memory cards having solid-state memory elements as recording media, and record and play back images captured using solid-state image sensors such as a CCD and CMOS sensor have been extensively developed and have prevailed. Such image capture apparatus are required to enhance resolutions associated with capturing of still images and moving images and to speed up operations. To meet these demands, the frequency of a drive signal required to drive a solid-state image sensor included in a digital camera, and the drive frequencies for an analog signal processing circuit, A/D converter, and subsequent digital signal processing circuit are rapidly speeded up.
Also, simple and convenient image capture operations that suffer fewer failures in various image capture scenes are further demanded. For this purpose, in order to follow a quickly moving object in, for example, a sports scene, or to attain image stabilization in an indoor image capture operation under the low illumination, higher shutter speeds are adopted. In order to allow image capture operations in places such as museums and aquariums where strobe image capture operations are prohibited, image capture apparatus are demanded to have still higher sensitivities.
An output of an image sensor includes noise of column offset components, which appear as vertical stripe-like noise caused by its structure. Such noise will be referred to as column offsets hereinafter. For example, vertical stripe noise due to defects of vertical transfer registers in a CCD sensor, a smear phenomenon which occurs at the time of incidence of strong light, and the like are known. Also, an X-Y address type sensor represented by a CMOS sensor generally has a structure for reading out signals for each selected row from photoelectric conversion elements, which are arranged in matrix, via each of vertical output lines which are common to respective rows and are different for respective columns. For this reason, column offsets readily occur due to variations of element characteristics that are different for respective columns.
FIG. 13 shows a basic circuit arrangement associated with a read-out circuit for one pixel in a general image sensor (CMOS sensor). Referring to FIG. 13, a photodiode 901 accumulates a light signal charge, and a transfer transistor 902 transfers the light signal charge accumulated on the photodiode to a floating diffusion 904. A reset transistor 903 resets the light signal charge accumulated on the photodiode, and the floating diffusion 904 converts the light signal charge into an FD potential. A pixel source-follower 905 reads out the FD potential onto a vertical output line connected to a column amplifier. Since vertical output lines and column amplifiers arranged for respective columns have different characteristic variations for respective columns, column offsets are generated.
The output of the image sensor includes various other noise generation factors: pixel defect noise generated due to the photodiode, reset noise generated due to the reset transistor, and 1/f noise and RTS noise generated due to the pixel source-follower. The reset noise is noise generated when the reset transistor is enabled and is then disabled by supplying a predetermined reference voltage, and can be removed by a state-of-the-art technique such as correlated double sampling (CDS circuit). The 1/f noise and RTS noise are both random noise components generated in processes of trapping and releasing electrons at an interface level of the pixel source-follower. Since the power spectrum density of the 1/f noise is inversely proportional to the frequency, and has a larger power at a lower frequency, the CDS circuit can largely reduce the 1/f noise. However, since the RTS noise is generated to have unspecified time intervals, it remains because the CDS circuit cannot remove it. The pixel defect noise is dark current noise due to an impurity mixed in the photodiode, and may become white spot noise with a very large level depending on the temperature and light signal charge accumulation time. The pixel defect noise also remains since the CDS circuit cannot remove it.
Conventionally, Japanese Patent Laid-Open No. 7-67038 has proposed the following technique. That is, in order to detect and cancel column offsets superposed on an image capture signal, a storage unit, which stores image data for one horizontal period, is arranged, and stores optical black pixels in a vertical direction of the solid-state image sensor by integrating them for a horizontal period. Then, the stored image data for one horizontal period is subtracted from effective pixel data, thereby removing the superposed column offsets.
As another related art, Japanese Patent Laid-Open No. 2006-25148 has proposed the following method. In this method, the influence of defective pixels that exceed a predetermined threshold level is removed from optical black pixels in the vertical direction of the solid-state image sensor, and column offsets are detected, thus enhancing the detection precision of the column offsets.
Furthermore, as a method of detecting column offsets superposed on an image capture signal, the following method has been conventionally examined. That is, image storage units, which store image data for a plurality of horizontal periods, are arranged, and store all optical black pixels in the vertical direction of the solid-state image sensor, and image processing such as a two-dimensional filter is applied to remove noise. Thus, the detection precision of column offsets is enhanced.
As described above, as an image capture apparatus gains still higher sensitivities, a technique for detecting column offsets with higher precision and removing them from an image capture signal is indispensable. However, the example of Japanese Patent Laid-Open No. 7-67038 does not consider any influence of noise other than the column offsets included in a column offset detection region. The example of Japanese Patent Laid-Open No. 2006-25148 has proposed elimination of the influence of defective pixels included in the column offset detection region, but it does not present any practical circuit arrangement and control method required to separate column offset components and other noise components included in a column offset detection region with high precision upon implementation of the invention. Also, this example does not embody any signal processing algorithm.
The method of providing the image storage units (RAMs) used to store image data for a plurality of horizontal periods and applying two-dimensional noise processing using these RAMs having relatively large capacities is very effective as a means for enhancing the detection precision of column offsets. However, this method results in an increase in circuit scale, and leads to high cost, large consumption power, and a bulky apparatus.